Current fuel cell membrane electrode assemblies (MEAs) suffer from incompatibilities between the materials used for membranes and electrodes, especially when non-traditional membranes (e.g., sulfonated polysulfone) are used with conventional Nafion®-bonded electrodes. These material property incompatibilities include differences in water uptake, electro-osmotic drag, and adhesion (chemical composition). The present invention method was the result of isolating the factors that affect performance and durability, and optimizing the method for producing advanced MEAs taking those factors into account. The present invention method produces MEAs that comprise multiple layers of polymer electrolytes that exhibit tunable properties leading to improved performance, properties, and durability under a wide range of fuel cell operating conditions, to include use in direct methanol and hydrogen fuel cells.
Polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), a subset of PEMFC, have been the center of attention for over a decade as possible candidates for next generation energy conversion devices. PEMFC and DMFC are currently being developed for a number of different applications. Some of the most important challenges for PEMFC construction methods are to reduce the membrane cost, increase durability, increase the operating temperature range, and increase conductivity at low levels of relative humidity (RH). The main challenge concerning DMFCs is to reduce the methanol crossover from anode to cathode, while maintaining high conductivity. Methanol crossover adversely affects the cell by lowering the cell voltage due to a mixed potential effect at the cathode (lower power density and efficiency) and lowering fuel utilization (lower efficiency).
The current state of the art perfluorinated sulfonic acid proton exchange membrane, Nafion®, is not only costly but also has a tendency to creep (limiting its durability, especially at high temperature), poor conductivity under dry conditions, and inherently high methanol permeability. As a consequence, significant effort has been spent developing alternative hydrocarbon based proton exchange membranes, which are less expensive, have higher glass transition temperatures and lower methanol permeability. Issues involving conduction at low RH are also being extensively studied, but materials that have adequate conduction for most applications under these conditions have not been found. Still, the present invention fabrication method and techniques presented are applicable to such systems once materials with the requisite properties are developed.
Many polymers [McGrath, et.al, U.S. Patent Application No. 20020091225, 2002, Koyama et. al, U.S. Pat. No. 6,670,065, 2003, L. Jorissen, et. al. J. Power Sources, 105, 267, 2002, K. Miyatake, et. al. Macromolecules, 37, 4961, 2004] have been identified that have promising properties for use in fuel cell systems, however, use of membranes other than perfluorinated sulfonic acid polymers have shown little or no performance improvement in fuel cell testing. In other words, anticipated performance improvements based on membrane properties have not been realized in functioning devices. A primary barrier to the successful integration of alternative polymeric membranes into high performance membrane electrode assemblies is attributed to minimizing interfacial resistance loss and interfacial delamination between the membrane and the electrode under fuel cell operating conditions.
The present invention allows for the incorporation of alternative polymers in fuel cell systems, while maintaining robust, high performance membrane electrode assemblies, and improving long-term cell performance (power density and/or fuel efficiency) and durability.
Therefore, in accordance with the present invention, a membrane coating fabrication method and consequent fuel cell membrane have been developed to overcome performance degradation arising from interfacial resistance due to the dimensional mismatch between membrane and electrode materials. The present invention can be applied to direct methanol and hydrogen fuel cells using proton exchange membrane especially for situations where the chemical and/or water swelling differences between the electrode and membrane are dramatic.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.